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Previously (Physical Properties of Haloalkanes), we learned that haloalkanes contain a polarized C-X bond, leaving a carbon that is partially positive and a halogen that is partially negative.
Examples of Negatively Charged and Neutral Nucleophiles
Now let's look at two actual examples of these two general equations. In the first reaction shown below, the negative nucleophile, hydroxide, reacts with methyl iodide. Hydroxide takes the place of the leaving group, iodide, forming neutral methanol and an iodide ion. This reaction is the same as the first type of nucleophilic substitution shown above. In the second reaction shown below, the nuetral nucleophile, ammonia, reacts with iodoethane. Ammonia takes the place of the leaving group, iodide, forming the positively charged product, ethylammonium iodide, and an iodide ion. This reaction is the same as the second type of nucleophilic substitution shown above.
Figure 2. SN2 reaction of methyl chloride and hydroxide ion (left) and ammonia reaction with HCl (right).
Next section: Using Electron-Pushing Arrows